Refrigerating cycle

ABSTRACT

In a freezing cycle that utilizes a supercritical fluid as its coolant and employs an internal heat exchanger that performs heat exchange on the coolant on the outlet side of a gas cooler and on the intake side of a compressor, a means for adjustment that adjusts the quantity of heat exchange performed by the internal heat exchanger ( 4 ) is provided. The means for adjustment is constituted of a bypass passage ( 9 ) that bypasses the internal heat exchanger ( 4 ) and a flow-regulating valve ( 10 ) that adjusts the coolant flow rate in the bypass passage ( 9 ). The flow-regulating valve ( 10 ) is constituted of an electromagnetic valve, the degree of openness of which is determined based upon information with respect to the cycle state, or a bellows regulating valve that operates in correspondence to the pressure on the high-pressure side. Alternatively, the means for adjustment may perform adjustment by varying the passage length over which heat exchange is performed by the internal heat exchanger ( 4 ). Good cycle efficiency is achieved by maintaining the optimal high-pressure through cycle balance control. The freezing cycle can be temporarily protected against excessively high-pressure or excessively high discharge temperature at the compressor.

TECHNICAL FIELD

The present invention relates to a freezing cycle achieved by using asupercritical fluid as a coolant, and more specifically, it relates to afreezing cycle provided with an internal heat exchanger that performsfurther heat exchange on the coolant at the intake side of a compressorand again at the outlet side of a gas cooler that cools down the coolantthat is at high-pressure, having been raised in pressure by thecompressor.

BACKGROUND TECHNOLOGY

A great deal of interest is focused on a freezing cycle that uses carbondioxide (CO₂) as one of the non-freon freezing cycles proposed asalternatives to a freezing cycle (freon cycle) that utilizes freon.While a freon cycle in the prior art requires a liquid reservoir such asa liquid tank to be provided in the high-pressure line in order toabsorb fluctuations of the load and leaking of the coolant gas occurringover time, a CO₂ cycle, in which the temperature on the high-pressureside exceeds the critical point (31.1° C.), unlike in the freon cycle,does not allow a liquid tank to be provided in the high-pressure line,thus necessitating an accumulator to be provided on the downstream siderelative to the evaporator.

As a result, since the liquid reservoir is provided on the downstreamside relative to the evaporator, superheat control such as that adoptedin a freon cycle cannot be implemented, and instead, a system that iscapable of controlling the high-pressure must be provided.

In addition, since the freezing capability and the COP (coefficient ofperformance: freezing effect/compressor work) of a CO₂ cycle areinferior to those achieved by a freon cycle, the cycle structure asillustrated in Japanese Examined Patent Publication No. H 7-18602 may beadopted to improve the freezing capability and the COP.

To explain this cycle structure in reference to FIG. 5, a freezing cycle1 that utilizes CO₂ is provided with a compressor 2 that raises thepressure of a coolant, a radiator 3 that cools down the coolant, aninternal heat exchanger 4 that performs heat exchange for coolantflowing through a high-pressure line and a low-pressure line, anexpansion valve 5 that reduces the pressure of the coolant, anevaporator 6 that evaporates and gasifies the coolant and an accumulator7 that achieves gas/liquid separation for the coolant flowing out of theevaporator. In this cycle, the coolant in a supercritical state with itspressure having been raised at the compressor 2 is cooled down by theradiator 3 and is further cooled by the internal heat exchanger 4 beforeit enters the expansion valve 5. The pressure of the coolant thus cooledis reduced at the expansion valve 5 and thus the coolant becomes moiststeam. After the coolant is evaporated at the evaporator 6, gas/liquidseparation is achieved by the accumulator 7, and then heat exchange withthe high-pressure side coolant is performed by the internal heatexchanger 4 so that the coolant becomes heated before it is returned tothe compressor 2.

These changes in the state of the cycle are as indicated as A→B→C→D→E→F→A in the Mollier diagram in FIG. 6, with the coolant indicated bypoint A becoming compressed at the compressor 2 to becomehigh-temperature, high-pressure coolant in the supercritical stateindicated by point B, the high-temperature, high-pressure coolant cooleddown to point C by the radiator 3 and further cooled down to point D bythe internal heat exchanger 4. Then its pressure is reduced at theexpansion valve 5 and the coolant becomes moist steam at a lowtemperature and a low pressure, as indicated by point E. Next, itbecomes evaporated and gasified at the evaporator 6 before reachingpoint F. The coolant having passed through the evaporator 6 is furtherheated by the internal heat exchanger 4 up to point A, and then iscompressed again by the compressor 2.

Thus, the cycle provided with the internal heat exchanger 4 achieves afreezing effect which is greater by the enthalpy difference betweenpoint E and point E′ compared to the freezing effect achieved by a cyclewithout the internal heat exchanger 4 (F-B′-C-E′-F), and since the workperformed by the compressor (the enthalpy difference between point A andpoint G) does not fluctuate greatly whether or not the internal heatexchanger 4 is provided, the COP can be increased by providing theinternal heat exchanger 4.

It is known that the freezing capability and the COP of a CO₂ cycle areaffected by high-pressure and that the COP is at its best at a certainpressure level (10˜15 MPa). For instance, in the summer when thetemperature of the coolant at the outlet of the gas cooler reachesapproximately 40° C., there is a high-pressure β which allows the COP toreach a maximum value a as shown in FIG. 7.

In addition, while the presence of the internal heat exchanger 4contributes to improving the COP as described above, it is known thatthere is an optimal value for the heat exchange quantity that allows theCOP to reach its maximum value as shown in FIG. 8.

Accordingly, an object of the present invention is to provide a freezingcycle utilizing a supercritical fluid as a coolant and provided with aninternal heat exchanger to perform heat exchange on the coolant at theoutlet side of a gas cooler and at the intake side of a compressor,which is capable of achieving good cycle efficiency by maintaining anoptimal high-pressure through cycle balance control. Another object ofthe present invention is to provide a freezing cycle which can betemporarily protected against excessively high-pressure or excessivelyhigh discharge temperature at the compressor.

DISCLOSURE OF THE INVENTION

In order to achieve the objects described above, the freezing cycleaccording to the present invention, which uses a supercritical fluid asa coolant comprises a compressor that raises the pressure of thecoolant, a gas cooler that cools down the coolant whose pressure hasbeen raised at the compressor, an internal heat exchanger that performsheat exchange on the coolant at the outlet side of the gas cooler and atthe intake side of the compressor, a means for pressure reduction thatreduces the pressure of the coolant supplied from the gas cooler via theinternal heat exchanger and an evaporator that evaporates the coolantwhose pressure has been reduced by the means for pressure reduction. Itadopts a cycle structure in which the coolant flowing out of theevaporator is returned to the compressor via the internal heatexchanger, and is characterized in that it is provided with a means foradjustment that adjusts the quantity of heat exchange performed at theinternal heat exchanger.

Thus, the high-temperature, high-pressure coolant having entered asupercritical state with its pressure raised at the compressor is thencooled by the gas cooler and is further cooled by the internal heatexchanger before it is led to the means for pressure reduction where itspressure is reduced until it becomes low-temperature, low-pressure moiststeam. After it is evaporated and gasified at the evaporator, it entersthe internal heat exchanger where its heat is exchanged with the heat ofthe high-pressure side coolant, and then it is supplied to thecompressor so that its pressure can be raised again. In this type ofcycle, in which the high-pressure line operates in the supercriticalrange, if the high-pressure is caused to fluctuate by the external airtemperature or the cooling load, the freezing effect willcorrespondingly fluctuate. However, by adjusting the quantity of heatexchange performed by the internal heat exchanger with the means foradjustment, the high-pressure is maintained at an optimal level, therebymaking it possible to achieve the maximum cycle efficiency.

While a fluid such as CO₂ with a critical temperature in the vicinity ofroom temperature is used as the supercritical fluid and the cyclestructure is provided with, at least, a compressor, a gas cooler, aninternal heat exchanger, a means for pressure reduction and anevaporator as minimum requirements, the structure may be furtherprovided with an accumulator on the coolant downstream side relative tothe evaporator or an oil separator between the compressor and the gascooler.

An effective structure that may be adopted in the means for adjustmentis constituted of a bypass passage that bypasses the internal heatexchanger and a flow-regulating valve that adjusts the coolant flow ratein the bypass passage. The flow-regulating valve provided at the bypasspassage may be constituted of an electromagnetic valve, the degree ofopenness of which is determined based upon information regarding thecycle state, or a bellows regulating valve that operates incorrespondence to the pressure in the high-pressure line. While thebypass passage may be provided in the high-pressure line, it is moredesirable to provide it in the low-pressure line from the freezing cycledesign aspect.

With the means for adjustment structured as described above, the flowrate of the coolant flowing through the internal heat exchanger isadjusted by controlling the flow rate of the coolant flowing through thebypass passage and, as a result, the high-pressure can be set to anoptimal level by varying the quantity of heat exchange performed by theinternal heat exchanger.

Instead of adjusting the flow rate in the bypass passage, the means foradjustment may perform adjustment by varying the length of the passageover which heat exchange is performed by the internal heat exchanger.With the means for adjustment structured as described above, thequantity of heat exchange performed by the internal heat exchanger isadjusted and likewise, the cycle balance is controlled, by varying therange over which heat exchange is achieved between the high-pressureside coolant and the low pressure side coolant even when the flow rateof the coolant flowing into the internal heat exchanger remainsconstant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a structural example of a freezing cycle according tothe present invention;

FIG. 2 is a schematic flowchart of an electromagnetic controlimplemented by a controller in FIG. 1;

FIG. 3 illustrates another structural example that may be adopted tocontrol a coolant flow rate in a bypass passage shown in FIG. 1;

FIG. 4 illustrates yet another structural example that may be adopted tocontrol the quantity of heat exchange performed by the internal heatexchanger shown in FIG. 1;

FIG. 5 illustrates the structure of a freezing cycle in the prior art;

FIG. 6 presents a Mollier diagram of the freezing cycle shown in FIG. 5;

FIG. 7 is a characteristics diagram illustrating the relationshipbetween the high-pressure in the freezing cycle provided with aninternal heat exchanger shown in FIG. 5 and its COP; and

FIG. 8 is a characteristics diagram illustrating the relationships amongthe quantity of heat exchange performed by the internal heat exchangershown in FIG. 5, the discharge pressure of a compressor, the dischargetemperature at the compressor, the freezing capability of the cycle andthe COP.

THE BEST MODES FOR CARRYING OUT INVENTION

The following is an explanation of preferred embodiments of the presentinvention given in reference to the drawings.

In FIG. 1, a freezing cycle 1 comprises a compressor 2 that compresses acoolant, a gas cooler 3 that cools down the coolant, an internal heatexchanger 4 that performs heat exchange on the coolant in thehigh-pressure line and the coolant in the low-pressure line, anexpansion valve 5 that reduces the pressure of the coolant, anevaporator 6 that evaporates and gasifies the coolant and an accumulator7 that achieves gas-liquid separation of the coolant.

In this freezing cycle 1, a passage extending from the compressor 2 tothe inflow side of the expansion valve 5, achieved by connecting thedischarge side of the compressor 2 to a high-pressure passage 4 a of theinternal heat exchanger 4 via the gas cooler 3 and connecting theoutflow side of the high-pressure passage 4 a to the expansion valve 5,constitutes a high-pressure line 8 a. In addition, the outflow side ofthe expansion valve 5 is connected to the evaporator 6 and the outflowside of the evaporator 6 is connected to a low pressure side passage 4 bof the internal heat exchanger 4 via the accumulator 7. A passageextending from the outflow side of the expansion valve 5 to thecompressor 2 achieved by connecting the outflow side of the low pressurepassage 4 b to the intake side of the compressor 2 constitutes alow-pressure line 8 b.

In this freezing cycle 1, CO₂ is utilized as the coolant, and thecoolant compressed by the compressor 2 enters the radiator 3 as ahigh-temperature, high-pressure coolant in a supercritical state, toradiate heat and become cooled. Then, it is further cooled down throughheat exchange with the low temperature coolant in the low-pressure line8 b at the internal heat exchanger 4, and is supplied to the expansionvalve 5 without becoming liquefied. Next, its pressure is reduced at theexpansion valve 5 until it becomes low-temperature, low-pressure moiststeam, and then becomes evaporated and gasified at the evaporator 6through heat exchange with the air passing through the evaporator 6.Subsequently, the coolant undergoes gas-liquid separation at theaccumulator 7 and the gas-phase coolant alone is guided to the internalheat exchanger 4 where it undergoes heat exchange with thehigh-temperature coolant in the high-pressure line 8 a before it isreturned to the compressor 2.

In addition, a bypass passage 9 which bypasses the internal heatexchanger 4 is provided in the low-pressure line 8 b in the freezingcycle 1. Namely, one end of the bypass passage 9 is connected betweenthe accumulator 7 and the internal heat exchanger 4 and the other end isconnected between the internal heat exchanger 4 and the compressor 2 sothat the gas-phase coolant resulting from the separation achieved at theaccumulator 7 is directly delivered to the compressor 2.

Furthermore, a flow-regulating valve 10 that adjusts the flow rate ofthe coolant flowing through the bypass passage 9 is provided at thebypass passage 9. The flow-regulating valve 10 may be constituted of,for instance, an electromagnetic valve, the degree of openness of whichis varied by a stepping motor 10 a, and its degree of openness isautomatically controlled by a controller 11.

The controller 11, which comprises a central processing unit (CPU), aread only memory (ROM), a random access memory (RAM), an input/outputport (I/O) and the like (not shown), is provided with a drive circuitfor driving the stepping motor 10 a of the flow-regulating valve 10 andprocesses various signals related to the cycle state in conformance to aspecific program provided in the ROM.

In other words, the controller 11 engages in the processing illustratedin FIG. 2, in which a pressure signal from a pressure sensor 12 thatdetects the discharge pressure at the compressor 2, a signal from adischarge temperature sensor 13 that detects the discharge temperatureat the compressor 2 and a signal from an evaporator temperature sensor14 that detects the load applied to the evaporator 6 as, for instance,the coolant temperature at the outlet of the evaporator are input (step50), an optimal pressure that allows the COP to reach the maximum valueis calculated based upon the signals, a decision is made as to whetheror not the high-pressure has risen to a level in the danger zone and adecision is made as to whether or not the discharge temperature hasrisen to a dangerous level (step 52) and the degree of openness of theelectromagnetic valve is determined based upon the results obtained instep 52 to implement drive control on the degree of openness of theflow-regulating valve 10 so that the desired degree of openness isachieved (step 54).

In the structure described above, if it is necessary to achieve themaximum COP, for instance, a decision can be made with respect to thequantity of heat exchange to be set for the internal heat exchanger 4 toachieve the optimal discharge pressure that allows the COP to reach themaximum value as indicated through the relationships in FIGS. 7 and 8,and thus, the degree of openness of the flow-regulating valve 10 iscontrolled to achieve this heat exchange quantity.

Furthermore, in addition to maintaining the most desirable operatingstate, the cycle can be temporarily protected by adjusting the coolantflow rate in the bypass passage 9 with the flow-regulating valve 10 ifthe pressure on the high-pressure side has risen to a level in thedanger zone due to a fluctuation of the load or if the dischargetemperature has risen to an excessive degree.

In more specific terms, if the high-pressure detected by the pressuresensor 12 has risen to a level in the danger zone due to a fluctuationof the load or the like, the flow-regulating valve 10 is closed to stopthe coolant from flowing into the bypass passage 9 so that the quantityof heat exchange performed by the internal heat exchanger 4 isincreased. As indicated by the characteristics presented in FIG. 8, byincreasing the quantity of heat exchange performed by the internal heatexchanger, the discharge pressure (indicated by the ) can be lowered.

In addition, if the discharge temperature detected by the dischargetemperature sensor 13 has risen to a level in the danger zone due to afluctuation of the load or the like, the degree of openness of theflow-regulating valve 10 is increased to increase the flow rate of thecoolant flowing into the bypass passage 9 so that the quantity of heatexchange performed by the internal heat exchanger 4 is reduced. Asindicated by the characteristics presented in FIG. 8, by reducing thequantity of heat exchange performed by the internal heat exchanger 4,the discharge temperature (indicated by the ▴) is lowered.

By changing the quantity of heat exchange performed by the internal heatexchanger 4 with the flow-regulating valve 10, the cycle balance can becontrolled freely to maintain the optimal high pressure so that themaximum cycle efficiency is achieved and also to temporarily protect thecycle if the pressure on the high-pressure side or the dischargetemperature has risen to an excessive degree. As a result, control,which is implemented in correspondence to the heat load can beimplemented in a freezing cycle that uses a supercritical fluid, as analternative to the superheat control implemented in a freon cycle in theprior art.

FIG. 3 illustrates another structural example that may be adopted toimplement control on the bypass flow rate. In this example, theflow-regulating valve 10 is constituted of, for instance, a bellowsvalve, the degree of openness of which is adjusted in correspondence tothe discharge pressure at the compressor 2, and the degree of opennessof the bypass passage is reduced as the high-pressure rises to increasethe flow rate of the coolant flowing into the internal heat exchanger 4.By adopting this structure, since the pressure on the high-pressure sideis fed back at all times to determine the coolant flow rate at thebypass passage 9, the quantity of heat exchange performed by theinternal heat exchanger 4 can be adjusted to sustain the pressure on thehigh pressure side at the optimal level at all times and, likewise, toachieve the maximum cycle efficiency even when the cooling load or thelike has fluctuated.

It is to be noted that while the bypass passage 9 shown in FIGS. 1 and 3may instead be provided in the high-pressure line 8 a so as to connectthe outlet side of the gas cooler 3 and the intake side of the expansionvalve 5, it is more desirable to provide it in the low-pressure line 8b, as illustrated in the figures, so as to connect the outlet side ofthe accumulator 7 and the intake side of the compressor 2.

This depends on reasons of {circle around (1)} if the bypass passage isprovided in the high-pressure line 8 a, a great quantity of high-densitygas concentrates in the high-pressure line 8 a to raise the pressure atthe low-pressure line 8 b greatly when the cycle operation stops, atwhich point the pressure over the entire cycle achieves equilibrium. If,on the other hand, the bypass passage is provided in the low-pressureline 8 b, the coolant density within the bypass passage is lower, evenwhile the volumetric capacity of the entire cycle remains the same, sothat the equilibrium pressure at the time of cycle stop can be reduced;{circle around (2)} it is necessary to reduce the volumetric capacity ofthe cycle and, in particular, the volumetric capacity of thehigh-pressure side, in order to reduce the volumetric capacity of theaccumulator 7 provided on the low pressure side; {circle around (3)}while it is necessary to ensure that the flow-regulating system iscapable of withstanding a high level of pressure within the range of10˜15 MPa to which the pressure on the high-pressure side rises whenproviding the bypass passage on the high-pressure side to adjust theflow rate, an existing device can be utilized if the bypass passage isprovided in the low-pressure line 8 b; and so on.

FIG. 4 illustrates another example of the means for adjustment providedto adjust the quantity of heat exchange performed by the internal heatexchanger 4, and the following explanation will mainly focus ondifferences from the previous example with the same reference numbersassigned to identical components to preclude the necessity for repeatedexplanation thereof.

In the freezing cycle 1, a passage 15 through which the coolant flowsfrom the accumulator 7 into the internal heat exchanger 4 branches intoa plurality of branch passages (e.g., 3 passages) 15 a, 15 b and 15 c.The first branch passage 15 a is connected so that the coolant isallowed to flow through the entire low pressure passage 4 b of theinternal heat exchanger 4, the second branch passage 15 b is connectedat a position at which the coolant flows into the low pressure passage 4b approximately ⅔ of the way along its length from the outflow end andthe third branch passage 15 c is connected at a position at which thecoolant flows into the low pressure passage 4 b approximately ⅓ of theway along its length from the outflow end. The individual branchpassages are opened/closed by flow-regulating valves 16 a, 16 b and 16 crespectively, each constituted of an electromagnetic valve. Theflow-regulating valves 16 a, 16 b and 16 c are driven/controlled by acontroller 11′.

This controller 11′, too, is capable of controlling the heat exchangequantity by receiving signals from the pressure sensor 12, which detectsthe discharge pressure at the compressor 2, discharge temperature sensor13, which detects the discharge temperature at the compressor 2 and theevaporator temperature sensor 14, which detects the load applied to theevaporator 6 as, for instance, the coolant temperature at the outlet ofthe evaporator, determining whether the individual flow-regulatingvalves 16 a, 16 b and 16 c are to be opened/closed in conformance to aspecific program provided in advance and changing the range of heatexchange (the passage length over which heat exchange is achieved)performed by the internal heat exchanger 4.

If it is necessary to maximize the COP, for instance, control wherebythe flow regulating valve corresponding to the branch passage that willmaximize the COP is selected and opened in conformance to therelationships illustrated in FIGS. 7 and 8 and the other flow-regulatingvalves are closed, is implemented in the structure described above.

In addition, if the high-pressure detected by the pressure sensor 12 hasrisen to a level in the danger zone due to a fluctuation of the load orthe like, the second and third flow-regulating valves 16 b and 16 c areclosed and the first flow-regulating valve 16 a is opened, to set thequantity of heat exchange performed by the internal heat exchanger 4 tothe maximum level. As indicated by the characteristics presented in FIG.8, by increasing the quantity of heat exchange performed by the internalheat exchanger 4, the discharge pressure can be lowered. Furthermore, ifthe discharge temperature detected by the discharge temperature sensor13 has risen to a level in the danger zone due to a fluctuation of theload or the like, the first and second flow-regulating valves 16 a and16 b, for instance, are closed and the third flow-regulating valve 16 cis opened to reduce the quantity of heat exchange performed by theinternal heat exchanger. As indicated by the characteristics presentedin FIG. 8, by reducing the quantity of heat exchange performed by theinternal heat exchanger 4, the discharge temperature can be lowered.

By adjusting the quantity of heat exchange performed by the internalheat exchanger 4 through the open/close control of the flow-regulatingvalves 16 a, 16 b and 16 c in this manner, the cycle balance can becontrolled and a high degree of cycle efficiency can be maintained. Atthe same time, if the pressure on the high pressure side or thedischarge temperature rises to an excessive degree, it can be lowered sothat the cycle is temporarily protected.

It is to be noted that while a plurality of branch passages are providedon the inflow side of the low pressure passage 4 b of the internal heatexchanger 4 to vary the heat exchange range (the passage length overwhich heat exchange is achieved) for the internal heat exchanger 4 inthe example described above, similar advantages may be achieved bybranching the outflow side of the low pressure passage 4 b into aplurality of passages to vary the length over which heat exchange isachieved or by providing a branch passage on the inflow side or theoutflow side of the high-pressure passage 4 a of the internal heatexchanger to vary the heat exchange range (the passage length over whichheat exchange is achieved). In addition, the number of such branchpassages should be determined by taking into consideration the requiredcontrol accuracy and the practicality and may be set at 2, 4 or more.

Furthermore, any of structures that allow the coolant flow rate or thepassage length over which heat exchange is performed, other than thestructures described above provided with the bypass passage and thebranch passages, may be adopted in the method of controlling thequantity of heat exchange performed by the internal heat exchanger.

Industrial Applicability

As explained above, according to the present invention, the freezingcycle utilizing a supercritical fluid as its coolant is provided with aninternal heat exchanger that performs heat exchange on the coolant onthe outlet side of the gas cooler and the coolant on the intake side ofthe compressor and a means for adjustment that adjusts the quantity ofheat exchange performed by the internal heat exchanger and, as a result,the cycle balance can be controlled with ease by varying the quantity ofheat exchange performed by the internal heat exchanger to control thehigh-pressure of the cycle, the discharge temperature at the compressor,the freezing capability of the cycle, the COP and the like.

Consequently, even when the cycle balance is shifted by the external airtemperature or the internal load, the high-pressure in the freezingcycle can be maintained at the optimal level by adjusting the quantityof heat exchange performed by the internal heat exchanger to achieve themaximum cycle efficiency. Moreover, in addition to maintaining theoptimal operating state, the cycle can be temporarily protected bysuppressing the high-pressure or the discharge temperature at thecompressor that has reached a level in the danger zone due to afluctuation of the load or the like through adjustment of the quantityof heat exchange performed by the internal heat exchanger.

What is claimed is:
 1. A freezing cycle utilizing a supercritical fluidas a coolant thereof, comprising: a compressor that raises the pressureof the coolant; a gas cooler that cools down the coolant whose pressurehas been raised by said compressor; an internal heat exchanger thatperforms heat exchange for the coolant on an outlet side of said gascooler and the coolant on an intake side of said compressor; a means forpressure reduction that reduces the pressure of the coolant suppliedfrom said gas cooler via said internal heat exchanger; and an evaporatorthat evaporates the coolant, whose pressure has been reduced by saidmeans for pressure reduction; wherein the coolant flowing out of saidevaporator is made to be returned to said compressor via said internalheat exchanger, characterized in that a means for adjustment is providedthat adjusts the quantity of heat exchange performed by said internalheat exchanger.
 2. A freezing cycle according to claim 1, wherein: saidmeans for adjustment is constituted of a bypass passage that bypassessaid internal heat exchanger and a flow-regulating valve that adjuststhe flow rate of the coolant flowing through said bypass passage.
 3. Afreezing cycle according to claim 2, wherein: said flow-regulating valveis constituted of an electromagnetic valve, the degree of openness ofwhich is determined in conformance to information regarding the cyclestate.
 4. A freezing cycle according to claim 3, wherein: the degree ofopenness of said electromagnetic valve is determined in conformance tosaid information with regard to the cycle state so as to achieve adischarge pressure at said compressor that results in a maximumcoefficient of performance.
 5. A freezing cycle according to claim 3,wherein: the degree of openness of said electromagnetic valve isdetermined in conformance with information regarding the cycle state sothat the quantity of heat exchange performed by said internal heatexchanger is increased by closing said electromagnetic valve when thepressure on the high-pressure side has risen to a level equal to orhigher than a specific pressure.
 6. A freezing cycle according to claim3, wherein: the degree of openness of said electronic valve isdetermined in conformance with information regarding the cycle state sothat the quantity of heat exchange performed by said internal heatexchanger is reduced by increasing the degree of openness of saidelectromagnetic valve if the discharge temperature at said compressorrises to a level equal to or higher than a specific temperature.
 7. Afreezing cycle according to claim 2, wherein: said flow-regulating valveis constituted of a bellows regulating valve, the degree of openness ofwhich is adjusted in correspondence with the pressure in a high-pressureline in said cycle.
 8. A freezing cycle according to claim 2, wherein:said bypass passage connects a downstream side of said evaporator and anintake side of said compressor.
 9. A freezing cycle according to claim1, wherein: said means for adjustment varies the passage length overwhich heat exchange is performed by said internal heat exchanger.
 10. Afreezing cycle according to claim 9, wherein: a means for changing saidpassage length is achieved by providing a plurality of branch passageson an inflow side or an outflow side of said internal heat exchanger,connecting said branch passages at different positions along saidpassage length within said internal heat exchanger, providingflow-regulating valves individually in said branch passages andselecting a flow-regulating valve to be opened among saidflow-regulating valves.
 11. A freezing cycle according to claim 10,wherein: a flow-regulating valve that allows the coefficient ofperformance to reach a maximum value is selected to be opened.
 12. Afreezing cycle according to claim 10, wherein: a flow-regulating valvethat increases said passage length when the pressure on thehigh-pressure side has risen to a level equal to or higher than aspecific pressure is selected to be opened.
 13. A freezing cycleaccording to claim 10, wherein: a flow-regulating valve that reducessaid passage length when the discharge temperature at said compressorhas risen to a level equal to or higher than a specific temperature isselected to be opened.
 14. A freezing cycle according to claim 1, 2 or9, wherein: the supercritical fluid is carbon dioxide.